1. Field of the Invention
The present invention relates generally to an apparatus for resetting the location of the reflective part of a diffractive optical modulator. More particularly, the present invention relates to an apparatus for resetting the location of the reflective part of a diffractive optical modulator, which resets the location of the reflective part of the diffractive optical modulator to an initial location thereof at a specific time point, thus increasing the ability to control the location of the reflective part.
2. Description of the Related Art
With the development of micro-technology, micro-machine (Micro-Electro-Mechanical System: MEMS) devices and small-sized apparatuses in which MEMS devices are included are attracting attention.
Recently, spatial optical modulators using such MEMS devices have been developed. An example of such spatial optical modulators is a diffractive optical modulator.
FIG. 1 is a perspective view showing a conventional open hole-based diffractive optical modulator.
Referring to FIG. 1, the open hole-based diffractive optical modulator includes a silicon substrate 121, an insulating layer 122, a lower reflective part 123, and a plurality of actuating elements 130a to 130n. 
The lower reflective part 123 is deposited on the silicon substrate 121 and is adapted to reflect incident light. Material used for the lower reflective part 123 may include a metal material, such as Al, Pt, Cr, or Ag.
An actuating element (as a representative, the actuating element designated by reference numeral 130a is described, but the remaining actuating elements are constructed in the same way) is formed in a ribbon shape, and includes a lower support 131a the bottom surfaces of opposite ends of which are respectively attached to opposite locations on the silicon substrate 121 deviating from the recess part of the silicon substrate 121, so that the center portion of the lower support 131a is arranged to be spaced apart from the recess part.
Piezoelectric layers 140 and 140′ are provided on the opposite ends of the lower support 131a, and the actuating force of the actuating element 130a is provided by the contraction or expansion of the provided piezoelectric layers 140a and 140a′. 
Each of the left and right piezoelectric layers 140a and 140a′ includes a lower electrode layer 141a or 141′ for supplying piezoelectric voltage, a piezoelectric material layer 142a or 142a stacked on the lower electrode layer 141a or 141a′ and adapted to contract or expand when voltage is applied to both surfaces of the piezoelectric material layer, thus generating a vertical actuating force, and an upper electrode layer 143a or 143a′ stacked on the piezoelectric material layer 142a or 142a′ and adapted to supply piezoelectric voltage to the piezoelectric material layer 142a or 142a′. When voltage is applied both to the upper electrode layer 143a or 143a′ and the lower electrode layer 141a or 141a′, the piezoelectric material layer 142a or 142a′ contracts or expands, thus causing vertical motion of the lower support 131a. 
Meanwhile, the lower support 131a is provided with an upper reflective part 150a deposited on the center portion thereof, and is provided with a plurality of open holes 151a1 and 151a2 formed therein.
Such open holes 151a1 and 151a2 allow light incident on the actuating element 130a to pass therethrough and to be incident on the lower reflective part 123 corresponding to the location at which the open hole 151a1 or 151a2 is formed, thus enabling light reflected from the lower reflective part 123 and light reflected from the upper reflective part 150a to form diffracted light.
In this case, the light, which is incident on the actuating element 130a while passing through the open hole 151a1 or 151a2 of the upper reflective part 150a, can be incident on the corresponding location of the lower reflective part 123. In the case where the distance between the upper reflective part 150a and the lower reflective part 123 is a multiple of an odd number of λ/4 when the wavelength of incident light is λ, the most diffracted light is generated.
A single upper reflective part 150a and a lower reflective part 123 corresponding thereto can form scanned diffracted light spots used to form the pixels of an image formed on a screen. Referring to FIG. 2 to describe this operation in detail, a diffractive optical modulator includes n upper reflective parts 150a to 150n, corresponding to an ath pixel, a bth pixel, a cth pixel, a dth pixel, an eth pixel, . . . , an nth pixel, which form an image formed on the screen. The diffractive optical modulator is described with reference to a single upper reflective part designated by reference numeral 150a. Light, reflected from the reflective surfaces 150a1, 150a2, and 150a3 of the upper reflective part 150a and light, passed through the open holes 151a1, 151a2, and 151a3 of the upper reflective part 150a (where 151a3 is the interval between the upper reflective part 150a and an upper reflective part 150b adjacent thereto) and reflected from the lower reflective part 123, forms diffracted light. This diffracted light forms scanned diffracted light spots corresponding to the pixels of the image formed on the screen.
That is, the upper reflective parts 150a to 150n respectively form scanned diffracted light spots corresponding to the pixels of the image formed on the screen, together with the reflective surface of the lower reflective part 123 corresponding to the upper reflective parts. The scanned diffracted light spots are aligned in a line, thus forming a scan line (in this case, a scan line is assumed to be composed of n scanned diffracted light spots corresponding to n pixels).
Meanwhile, in the above-described open hole-based diffractive optical modulator, when piezoelectric voltage is applied to the left and right piezoelectric layers, the displacement of each upper reflective part, caused by the actuating force generated by the piezoelectric layer, exhibits hysteresis characteristics, as shown in FIGS. 3A and 3B. Referring to FIG. 3A, when the piezoelectric voltage to be applied to the left and right piezoelectric layers increases from 0V up to Vmax, the displacement of the upper reflective part is changed along line A. When the piezoelectric voltage decreases from Vmax down to 0V, the displacement of the upper reflective part exhibits hysteresis characteristics progressing along line B. Further, lines A and B are curved lines rather than straight lines, and thus exhibit non-linearity.
The hysteresis characteristics of the diffractive optical modulator exhibit the displacement characteristics of FIG. 3B when the piezoelectric voltage to be applied increases up to the maximum voltage Vmax, at which the maximum displacement is reached, decreases from the maximum voltage Vmax down to a voltage less than Vmax, and increases again from that voltage up to the maximum voltage Vmax.
That is, referring to FIG. 3B, the displacement characteristics of line B4 are exhibited when the application voltage increases from 0V up to the maximum voltage Vmax, at which the maximum displacement is reached, gradually decreases from Vmax down to voltage V4, and subsequently increases from V4 up to the maximum voltage Vmax.
Further, the displacement characteristics of line B3 are exhibited when the application voltage gradually decreases from Vmax down to voltage V3 and subsequently increases from V3 up to the maximum voltage Vmax.
Further, the displacement characteristics of line B2 are exhibited when the application voltage gradually decreases from Vmax down to V2 and subsequently increases from V2 up to the maximum voltage Vmax.
Further, the displacement characteristics of line B1 are exhibited when the application voltage gradually decreases from Vmax down to V1 and subsequently increases from V1 to the maximum voltage Vmax.
Meanwhile, the hysteresis characteristics of the diffractive optical modulator are also exhibited even when the application voltage increases from the minimum voltage up to an arbitrary voltage, at which desired displacement is reached, and then gradually decreases from the arbitrary voltage down to 0V.
Such hysteresis characteristics of the diffractive optical modulator make it difficult to determine the voltage to be applied to move the upper reflective part to a desired location.
Meanwhile, the above-described diffractive optical modulator exhibits a creep phenomenon in which, even if the same drive voltage is applied, the reflective part has different initial locations.
Such hysteresis and creep phenomenon make it difficult to determine the voltage required to move the upper reflective part to a desired location, and a solution to overcome this difficulty is required.